A styrene-butadiene copolymer obtained by emulsion polymerization is known as rubber used for motorcar tires. However, said copolymer has a problem that motorcar tires comprising said copolymer are not satisfactory from a viewpoint of fuel cost saving, because the copolymer does not have sufficient impact resilience.
In order to obtain rubber having superior impact resilience, JP-B 5-46365 discloses a process, which comprises copolymerizing butadiene and styrene in a hydrocarbon solvent using an organolithium compound as an initiator, and a Lewis base such as ether as a microstructure-controlling agent.
Further, Japanese Patent No. 2540901 proposes a process, which comprises the step of reacting an alkali metal bound at the end of a diene polymer rubber with a specific acrylamide to obtain a modified diene polymer rubber having improved impact resilience.
Furthermore, JP-A 2002-128824 discloses a process, which comprises the step of reacting an alkali metal bound at the end of a diene polymer rubber with a specific amine to obtain a modified diene polymer rubber having improved impact resilience and processability.
However, in recent years, a level demanded for fuel cost saving of motorcar tires is higher from an environmental view point, and therefore, any of the above-mentioned copolymer rubbers can hardly satisfy such a demand.